A power source of an electronic device such as a television set, a smart phone, or a personal computer should stably supply a constant DC voltage. As this power source, a resonance type switching power-supply device is used.
A resonance type switching power-supply device uses a semiconductor device such as a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), an Insulated Gate Bipolar Transistor (IGBT), or a thyristor as a switching element, and temporarily converts an input DC voltage into an AC voltage by turning on-and-off the switching element. The converted AC voltage is converted into a stable DC voltage by passing through a transformer, a rectifier circuit, and a smoothing circuit in the order of them, and the stable DC voltage is output.
In this switching power-supply device, the switching frequency of the switching element is controlled according to the output voltage, whereby the output voltage is stabilized. As for the control range of the switching frequency, a frequency range higher than the resonant frequency of a resonant circuit which is made up of a capacitor and an inductor is regarded as a normal control range. In order to widen the control range of the output voltage as much as possible, there is a case where it is tried to bring the lower limit frequency of the switching frequency as close to the resonant frequency as possible. However, if there are variations in the characteristics of elements to make up circuits, in the above-mentioned case, so-called off-resonance, in which the switching frequency becomes lower than the resonant frequency outside the normal range, may occur.
In Japanese Patent Application Laid-Open No. 2009-112123, there is disclosed a switching power-supply device which reads data on the voltage between both ends of a switching element, and converts the data into numerical values, and compares the maximum value of the data with a predetermined value, thereby estimating an abnormal operation.
In Japanese Patent Application Laid-Open No. 2005-198456 and No. 2013-188099, there are disclosed switching power-supply devices which perform overcurrent protection by reversing the ON/OFF states of a pair of switching elements if off-resonance is detected by comparing a resonant current with a predetermined current value.